Electric / hybrid aircraft propulsion (merged)

But would it not make more sense to design a hybrid drive where the engine and the electromotor were both connected to the prop? I mean, the ICE basically always producing power, and the electromotor kicking in when needed. At take-off, or at go-around or another phase requiring high power. Imagine the wings covered with light, thin-film solar panels. Of course there need to be batteries on board, and that brings a weight penalty. But one thing is clear; the automotive industry is going to lead us to high performance and low weight batteries over the next years. A 155 HP Technify Diesel, or 170 HP Austro Diesel complemented with a say 70 HP electromotor makes for a nice amount of power. Yes, weight is an issue, but less so with the Technify..

In terms of safety this is interesting too. The electromotor should be sized such that the aircraft at least maintains altitude, or ekes out a shallow climb after take off when the ICE fails. In that case you would have essentially created the safety of a twin, without the asymmetric thrust problem. A lot better than a parachute.

Maybe not too late for Pipistrel to do something really innovative and do justice to their wonderful Panthera design.

Of course, in the long run all electric seems the way to go, once the batteries are there. But the above may be a good interim solution, including twin-like safety. Am I dreaming?

The Zoche aero-diesel follows that concept. The electric engine is part of the drive shaft and operates as a generator in cruise but can deliver extra power during takeoff and initial climb from batteries. That is a great concept, especially for seaplanes where takeoff performance requirements are about twice the cruise power.

The technologies are all available and work well. My electric car produces 75kW and more during regenerative braking, all with the same circuitry and battery as used for powering the motor. You could even charge your batteries during descent

I don’t quite see the point. In a car, you have a very high ratio of peak power versus average power. There it makes sense.

In an aircraft however, you take off at 100% and cruise at 65%, or for the normally aspirated crowd, at 100% of what the engine can do at altitude. So this means you’re not getting away with a smaller ICE.

Despite the article claiming the drive train to be “low weight”, it delivers less power than a Rotax 914 at almost twice the mass (100kg vs 60kg).

Unlike gas tanks, a batteries real condition cannot easily be checked quickly. So you risk losing a large percentage of your propulsion at the most critical phase of flight, during take off.

Also, the claims about silent takeoff are surely exaggerated, in many aircraft engine and prop noise are very roughly the same magnitude, so with any hybrid drive you still have the prop noise and for safety you still have to run the ICE during takeoff.

And, unlike gas tanks, the battery weights the same, whether full or empty.

But you could, in basic flight training, save some fossil energy by operating an engine at it’s efficiency peak all the time and boost delivered energy for the parts of the flight, where it’s needed. Doesn’t make much sense in a tourer, though.

In an aircraft however, you take off at 100% and cruise at 65%, or for the normally aspirated crowd, at 100% of what the engine can do at altitude. So this means you’re not getting away with a smaller ICE.

That is only true at low altitudes. Look at the Thielert/Technify C172. It has great cruise performance compared to the avgas Cessna but bad takeoff performance. A small efficient diesel with an electric boost could make it much better. Same for the DA40 — excellent cruise, bad takeoff.

The Centurion 1.7 does 135BHP takeoff power at 134kg mass. About twice the mass per BHP takeoff power than your typical gas engine, be it Rotax or Lyconti.

I don’t have exact data right now, but AFAIR the output power of the Thielert starts to drop off at around 10k ft, so if you want to fly above that, you still need a relatively big ICE.

I agree that the Thielert C172 and DA40 have bad takeoff performance, but I doubt you can really fix it by adding even more weight. Weight doesn’t matter much during cruise (being dominated by parasitic drag), but for takeoff run and climb it enters fairly linearly into the equation.

Motors are about 2-3 horsepower/kg so your 70hp will be about 20-35 kg for the motor, plus batteries. A 70 horsepower (50 kilowatt) battery capable of supplying 6 minutes at full power will need to supply 5 kilowatt hours and weigh in at perhaps 25 kilos. Of course you save some weight because you cut out the alternator and starter motor battery and the starter motor but it’s still going to lead to a substantial increase in weight. My guess is that it would be worth it on take off, but that you would lose range and payload.

It isn’t hard to tell when lithium cells are full as their voltage rises rapidly, but it can be hard to assess when cells are part-way full.

Making the plane able to maintain level flight on electric power alone probably won’t be worth it, unless you’re planning to carry enough batteries to eat into the payload very substantially. 10 minutes of cruise at low power + glide from 3000 feet would rarely get me to an airport where I fly. What a bit of power could do, would be to give me a lot more latitude to extend the glide in order to choose an optimal forced landing spot.

An advantage for a tourer could be enhanced climb rates at altitude. You’d have 70 horsepower available for a few minutes, even at flight level 1000. Whether or not the propeller could use it or not is another matter.

For a training plane, what I’d do would be to have things the other way round – you’d carry enough batteries for an hour and a half of flight, and a small avgas powered generator and enough fuel to give the plane a reasonable flight duration at low power/cruising speeds, so that you can do an average training flight and keep legal reserves for if you need to divert. Another possibility there though would be to use aluminium air batteries which have an even higher energy density but aren’t rechargeable. Any flight where you used them would get very expensive, but it would need a lot less maintenance and complexity than a hybrid system.

What I suspect we will see eventually is a hybrid akin to a Mueller skycar or if you like a tiltrotor quadcopter/hexacopter – enough battery power for just 5 minutes of vertical flight. Very small wings and whetted area for fast and efficient flight on low power. Petrol engine for range during the low-power cruise. Ballistic parachute for engine/electrical failure. If you didn’t need to consider stall speeds, the amount of wing you’d need could be reduced hugely.

If you talk about ‘Diesel and low mass’, you also need to put into the equation that you will carry less fuel for a given mission. Yes, the specific mass of AVTUR is a bit heavier, but still the mass on board is lower. Suppose that a Diesel C172 uses 5 USG/hr and its AVGAS counterpart 9 USG/hr, that means that for a 3 hour flight plus one hour reserve you’d depart with 62 kg of AVTUR vs 100 kg of AVGAS, a 38 kg difference at take-off.

The 5 USG vs 9 USG number must be from the Diamond/Thielert/SMA propaganda leaflet At Peak EGT, the diesel is max 30% more efficient than the avgas engine.

BTW, in the hybrid setup, you can charge your lithium batteries during flight with the ICE. It could do miracles for those aircraft underpowered for takeoff and allow designing different airframes. Due to the limited power available during takeoffs, airframes have to produce a lot of lift at low speeds which they don’t have to do if the engine has more power. You get better cruise efficiency with such a design. Lithium batteries can be anywhere and it’s a big advantage being able to freely distribute weight when designing an airframe.

It isn’t hard to tell when lithium cells are full as their voltage rises rapidly, but it can be hard to assess when cells are part-way full.

But how do you tell how much capacity the battery still has after a few charge/discharge cycles?

Nobody apparently has figured out how to do this in a laptop or mobile phone, so I’m not holding my breath, even if aviation lithium batteries have a slightly different chemistry and somewhat better quality.

Due to the limited power available during takeoffs, airframes have to produce a lot of lift at low speeds which they don’t have to do if the engine has more power.

That would likely mean much higher landing speed and thus energy, thus would run afoul CS23 and be a safety concern.

But you could design a more streamlined cowling if the relatively large ICE needn’t sit in front.